Shear strength, CBR, and interface interaction of pumice-modified clay reinforced with Geostrip: A large-scale laboratory study

Clayey soils are challenging in geotechnical engineering because of their high compressibility, moisture sensitivity, and low shear strength. This study investigates lightweight and sustainable pumice, locally termed Ponza, for stabilizing clayey soil collected from Al-Amiriya, Baghdad, Iraq. Pumice was incorporated at replacement ratios of 10%, 20%, 30%, 40%, and 50% by dry mass. For each mixture, the dry constituents were blended, mixed with water at the corresponding optimum moisture content, and compacted before testing; CBR specimens were then soaked for 72 h. The experimental program included modified Proctor compaction, soaked California Bearing Ratio tests, and large-scale direct shear tests under normal stresses of 25, 50, and 75 kPa for unreinforced and Geostrip-reinforced specimens. A 200-mm-wide shear box was selected to accommodate the 4.75–9.5 mm pumice particles and minimize particle-size and boundary effects. Increasing pumice content reduced the maximum dry density and produced an overall increase in optimum moisture content because of the lightweight and porous nature of pumice. CBR generally improved at higher pumice contents owing to greater particle interlocking and the development of a frictional load-bearing skeleton. The shear response gradually changed from cohesion to friction-controlled behavior. For unreinforced samples, the friction angle increased from 16.3° for natural clay to 31.0° for 50% pumice, and cohesion decreased from 19.69 kPa to 10.50 kPa. Average interface interaction coefficients of 0.94–0.99 are shown to be efficient at stress transfer and compatible with the clay-pumice mixtures and Geostrip reinforcement.